I lost a significant amount due to a manipulation, but instead of liquidating my position (although I was below the limit for a margin call), a second clearing occurred "due to high volatility" half an hour or an hour later. You can probably still find the announcement in the Twitter feed. Fireball quickly stopped doing this, probably he realised that this kind of ad-hoc manual interventions are not a good idea.
Additional clearings are normal when market conditions are rapidly changing. They are pre-announced via official Twitter channel (displayed on the site's frontpage). I did an additional clearing because the market went too much against spot market and when the rate returned back (thanks to arbitragers) I did the additional clearing to move trading range back. No margin calls happened during that period because that would be greatly unfair to our clients.